Previous work by these investigators have reported ischemic ECG changes following sudden strenuous exercise in subjects with patent coronary arteries. Other studies in our laboratory have shown that ischemia with patent coronaries is due to an imbalance between oxygen demands and available oxygen supply (coronary flow and oxygen content). The present studies are designed to determine the effects of acute exercise on total and regional coronary blood flow (electromagnetic flow meters and radioactive microspheres) and the adequacy of oxygen delivery to the myocardium, especially the subendocardium in chronically instrumented dogs. Coronary blood flow (CBF) adaptation at the onset of sudden, strenuous exercise will be studied in order to determine the mechanism responsible for the ischemic responses. We will modify the supply/demand imbalance during sudden exercise by (a) pharmacological dilation to enhance supply, and (b) beta-receptor blockade to reduce demands in order to see if the ischemic response can be avoided. Further studies will be conducted during progressive exercise to near-maximal levels to determine if the availability of coronary blood flow is a limiting factor for maximal cardiac performance. We will quantitate vasodilator reserve capacity, especially to the subendocardium, by measuring CBF with and without pharmacologic vasodilation. The effects of training will be studied to see if differences exist between adaptations to a health maintenance program (3 days/week, 30 min/day) and an athletic training program (5 days/week, 60 min/day); total and regional myocardial blood flow, myocardial performance and biochemistry will be studied.